Catharus ustulatus

Song

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Figure 1. Distribution of Swainson's Thrush in North and Middle America.

Adult Swainson's Thrush

This secretive denizen of forests and woodlands across the northern portion of the continent is perhaps best known by its flutelike, upwardly spiraling song that lingers well into the fading summer twilight. It is a Nearctic-Neotropical migrant that breeds as far north as Alaska and northern Canada and winters primarily in South America. One of the most characteristic birds of montane fir forests over much of its range, Swainson's Thrush fills a different niche in coastal California, where it inhabits riparian woodlands. These 2 groups, the "olive-backed" and coastal breeding "russet-backed" thrushes, also differ in morphology, in some aspects of their vocalizations, and in their wintering areas, with the russet-backed group wintering primarily in Middle America and the olive-backed in South America. Adults of both sexes often return to previous breeding sites, which may be an advantage in northern, higher-elevation forests where persistent snow packs foreshorten the breeding season.

Originally classified in the genus Turdus from specimens on the Columbia River in Washington and the Saskatchewan River in Canada, Swainson's Thrush is now included in the genus Catharus with the Veery (C. fuscescens) and Hermit (C. guttatus), Gray-cheeked (C. minimus), and Bicknell's (C. bicknelli) thrushes. Among these and other closely related, spotted North American thrushes, it forages higher off the ground than its relatives and uses more aerial, fly-catching techniques to obtain insect prey, a characteristic that earned it the name "mosquito thrush" in Maine (
Bent, A. C. 1949. Life histories of North American thrushes, kinglets, and their allies. U.S. Natl. Mus. Bull. no. 196.
Bent 1949). Eight forms of Swainson's Thrush have been described based on geographical differences in coloration, with six currently recognized as subspecies. The confused historical taxonomy of this species and currently recognized differences may yet prompt further reclassification.

Although Swainson's Thrush is still considered one of the most common birds of northern spruce-fir forests, populations are declining even where abundant, particularly in Alaska and the Northeast. In California, the breeding range of this species has contracted during the last century, and "the disappearance of the Swainson's Thrush from Yosemite Valley is one of the unsolved mysteries of Sierran ornithology" (
Marshall, J. T. 1988b. Birds lost from a giant sequoia forest during fifty years. Condor no. 90 (2):359-372.
Marshall 1988b: 367, citing Beedy and Granholm 1985). The trend is clear, but the reasons are not. Nests are not besieged by cowbirds, but predation rates are high, resulting in low nest-success rates overall, critically low in some locales. Loss of breeding habitat may be a contributing factor, including loss of mature conifer forest and forest fragmentation, but Swainson's Thrushes are relatively abundant in some early-successional habitats, such as conifer plantations; this fact clouds the forest-management issue. On the Pacific Coast, loss of riparian habitat to development and grazing have likely contributed to declines. The impact of habitat changes on wintering grounds is unclear.

Several aspects of this species' life history have been relatively well studied, primarily in the eastern United States: migration behavior and physiology (WY and others), physical and ecological separations from closely related thrushes (Dilger
Dilger, W. C. 1956a. Adaptive modifications and ecological isolating mechanisms in the thrush genera Catharus and Hylocichla. Wilson Bull. no. 68:171-199.
Dilger 1956a,
Dilger, W. C. 1956b. Hostile behavior and reproductive isolating mechanisms in the avian genera Catharus and Hylocichla. Auk no. 73:313-353.
Dilger 1956b,
Noon, B. R. 1981. The distribution of an avian guild along a temperate elevational gradient: the importance and expression of competition. Ecol. Monogr. no. 51:105-124.
Noon 1981), and diet and foraging ecology (
Beal, F. E. L. 1915a. Food habits of the thrushes of the United States. U.S. Dep. Agric. Bull. no. 280.
Beal 1915a,
Holmes, R. T. and S. K. Robinson. 1988. Spatial patterns, foraging tactics, and diets of ground-foraging birds in a northern hardwood forest. Wilson Bull. no. 100:377-394.
Holmes and Robinson 1988). Habitat selection has been described for this species in northeastern montane forests (e.g.,
Sabo, S. R. 1980. Niche and habitat relations in subalpine bird communities of the White Mountains of New Hampshire USA. Ecol. Monogr. no. 50:241-260.
Sabo 1980) and Alaska taiga (
Kessel, B. 1998. Habitat characteristics of some passerine birds in western North American taiga. Fairbanks, AK: Univ. of Alaska Press.
Kessel 1998), but similar information is lacking for other habitats. Other than observations of a few nests in Maine in the early 1900s (
Stanwood, C. J. 1913. The olive-backed thrush (Hylocichla ustulata swainsoni) at his summer home. Wilson Bull. no. 25:118-137.
Stanwood 1913), the breeding ecology of this species is not known, perhaps because nests can be so difficult to find. Recent studies in western coniferous forests (DEM and others), northeastern coniferous forests (K. McFarland and others), and riparian systems in California (T. Gardali, S. Small et al.) have focused on how habitat (especially nest sites) affects reproductive success. Combined with additional research at wintering sites, this information may help explain why this thrush has declined.